1. Field of the Invention
The present invention generally relates to a zero-current-transition (ZCT) technique suitable for three-phase inverter and rectifier applications and, more particularly, to an improvement on a family of existing products, namely, three-phase-soft-switching inverters and rectifiers.
2. Description of the Prior Art
Three phase inverters (devices which convert direct current to three-phase alternating current) and rectifiers (devices which convert three-phase alternating current to direct current) have gained increased attention in recent times. In particular, efficient operation of such devices are of critical in applications such as AC adjustable speed drives for so-called zero-emission vehicles (i.e., electric and hybrid combustion/electric automobiles). Other applications include three-phase power factor correction (PFC) rectifier for DC power distribution systems as well as general purpose AC drives, utility power systems and uninterrupted power supplies (UPS).
The basic concept of zero current transition (ZCT) techniques is to force the current of an outgoing switch in a PWM power converter to zero prior to its turn-off. By using the ZCT techniques, converters can achieve a higher switching frequency with reduced switching losses and fewer electromagnetic interference (EMI) problems. The ZCT techniques are very attractive in high-power three-phase inverters and rectifiers where the minority-carrier devices, such as insulated gate bipolar IGBTs, are the power devices.
The ZCT commutation is usually assisted by some kind of auxiliary circuitry. The ZCT techniques are expected to be helpful to both turn-on and turn-off transitions of the main switch. The auxiliary switches should be soft-switched. Meanwhile, the schemes should not cause high voltage, current, or thermal stress on the devices and components.
As shown in FIGS. 1A-B, in existing three-phase ZCT inverters and rectifiers, six auxiliary switches (Sx1-Sx6) and correspondingly six gate-drivers for the auxiliary switches are needed, resulting in severe cost, layout, and reliability penalties. The consideration made in choosing a topology is that the independent commutation for each main switch should be retained such that the conventional space-vector pulse width modulation (PWM) schemes for hard-switching inverters and rectifiers can be directly employed without modification, and a possible sub-harmonic problem can be avoided. The existing three-phase ZCT topology shown in FIGS. 1A-B has this desired xe2x80x9cpiggy-backxe2x80x9d feature, where each phase leg of the main circuit has a corresponding auxiliary circuit, including two auxiliary switches and one resonant tank consisting of an inductor 10 and a capacitor 12. In total there are six (Sx1-Sx6) auxiliary switches in a three-phase ZCT inverter/rectifier. A number of three-phase ZCT techniques are known. They actually have the same circuit topology as shown in FIGS. 1A-B, but employ different soft-switching schemes, resulting in different operations and features.
Besides the topology shown in FIG. 1, there are a few other ZCT-types topologies proposed, but they are not suitable for three-phase inverter/rectifier applications. For instance, topologies have been proposed that require a middle-point tapped resonant inductor to be in series with the main switch. In three-phase systems, such as AC motor drives, the load itself is inductive; consequently, it is impossible to insert a resonant inductor in the main power path.
In short, the topology shown in FIGS. 1A-B so far is the most suitable for the three-phase inverter and rectifier applications. However, it requires too many components- six auxiliary switches (and correspondingly six additional gate-drivers), resulting in severe cost, layout and reliability penalties.
New ZCT topologies are presented for three-phase inverter and rectifier applications. Compared to existing three-phase ZCT techniques, the number of auxiliary switches is reduced from six to three, while not altering the necessary device rating. Correspondingly, the number of gate-drivers for the auxiliary switches is also reduced to three. Meanwhile, the assets of the existing three-phase ZCT techniques are still retained, i.e., all the main switches and the auxiliary switches are turned on and turned off under zero-current conditions, and the independent communication for each main switch is achieved. The desired soft-switching features are achieved. Therefore, this invention will contribute to more cost-effective, reliable, and efficient high-performance three-phase inverters and rectifiers.